It is well known that, when used in olefin polymerization, in particular in propylene polymerization, a solid titanium based catalyst component comprising magnesium, titanium, a halogen and an internal electron donor as essential constituents exhibits a high catalytic activity and a high stereospecific property. As the internal electron donor compound develops, the catalyst for olefin polymerization has ceaselessly progressed. The catalysts using different internal electron donors may have different properties, for example, activity, hydrogen response property, the molecular weight distribution of the resulting polyolefin resin, and the like.
Patent applications CN 1436766A and CN 1552740A disclose esters of diol useful as internal electron donor in a catalyst for olefin polymerization. When used in propylene polymerization, a catalyst prepared by using the esters of diol as internal electron donor is characterized by a high polymerization activity and a broad molecular weight distribution of polymer. However, we have found that, when used in propylene polymerization, a spherical catalyst prepared by using the esters of diol as internal electron donor (using an adduct of magnesium dichloride and an alcohol as a carrier) exhibits a poor stereospecific property so that the resulting polypropylene has a low isotacticity.
Chinese patents CN1020448C and CN100348624C and Chinese patent application CN1141285A disclose diethers useful as internal electron donor. A catalyst using the diethers as internal electron donor has a relatively high activity and polymer isotacticity, and can give a polymer having a high isotacticity at a high yield even when no external electron donor is used. However, the polymer obtained by using such a catalyst in a single polymerization reactor process has a relatively narrow molecular weight distribution so that the application of the polymer is limited.
Some studies have improved the performance of a catalyst by improving the preparation process of the catalyst. The solid catalyst components obtained by using different preparation processes may be quite different from each other in microstructure so that the number of the active sites and the distribution of various active sites are different. As a result, the catalysts are quite different in performance. The improvements in catalyst preparation process include mainly the use of different magnesium dichloride carriers and different preparation processes and the optimization of the preparation conditions.
The enhancement of polymerization activity of a catalyst is all the while one of the most important study directions in the art. In recent years, as the demand for the superpure, low ash polypropylene increases, the demand for a catalyst, which can retain a high polymerization activity at a low alkyl aluminum concentration, is more and more imperious.